There must be a separate agreement unless you bought the shares using a margin loan. Just having a margin account is not sufficient.

I believe it's not just if you bought them with a margin loan, but rather if you have a margin loan with them as collateral. (ex. if I have stock A in my account, and I buy stock B using just margin, both A and B could be lent. Or if I have stock A, get cash from the broker as a margin loan, then A could be lent out)

But after a little research - looks like I'm going by what Scottrade does:

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